Pneumatic and hydraulic pressure displacement dredging systems employ pressure vessels which are lowered into an operating position immediately adjacent the bottom of a lake, pond, stream, or other body of water where dredging is required. A pressure vessel is first filled with material from the bottom by venting the vessel so that water and entrained solids flow into the vessel, by hydrostatic displacement, through an intake port in its base. Subsequently, air or some other operating fluid is introduced into the top of the vessel under high pressure to discharge the material, through an outlet port located near the vessel bottom, into a spoil pipe or other material discharge conduit. Subsequently, the high pressure fluid is exhausted from the vessel and bottom material again flows into the pressure vessel to begin another pumping cycle.
A pressure displacement dredging system using a single pressure vessel is subject to highly undesirable surges in operation, caused by the alternating filling and discharge cycles. Furthermore, a single-vessel pressure displacement dredge is slow and inefficient in operation because the duty cycle is only about fifty percent.
The inefficiency of a single vessel pump is avoided in the pump described in Callow U.S. Pat. No. 1,000,713, which incorporates two pneumatic pressure vessels operating in alternation in a single pumping system. When a first vessel is filled, a level sensor in that vessel closes its vent and opens a high pressure air inlet to begin discharging material. The level sensor also shuts off the high pressure air supply to a second vessel and opens its vent so that discharge of material from the second vessel is initiated and replaces the interrupted discharge from the first vessel.
In the Callow pumping system, however, surging can still occur during changeover from one pressure vessel to the other. If the vessels fill faster than they discharge, the changeover between vessels occurs without emptying the vessels and the cumulative effect leads to stalling of the system with both pressure vessels filled. If the vessels fill slower than they discharge, high pressure air may be wasted in each cycle while one vessel is empty but must wait for the other to fill, a generally inefficient arrangement.
Another pumping system, which provides some improvement over the Callow patent, is described in Stafford U.S. Pat. No. 2,669,941. Stafford provides level sensors at both the top and the bottom of two pressure vessels. When the material in a first vessel falls below its lower sensor, during discharge, the air inlet to the second vessel is opened and its vent is closed to initiate discharge. At the same time, the air inlet to the first vessel is closed; its vent is opened after a predetermined time delay. This effectively minimizes surging if one vessel fills before the other is completely discharged. On the other hand, if one vessel does not fill before the other is emptied, then changeover between vessels occurs with only a partially filled vessel available for continuing discharge. On each subsequent cycle, the vessels fill to progressively lower levels; the control cycles between the two vessels at an increasingly rapid rate that becomes highly inefficient and leads to an effective stallout, with excessive wear on the system. Moreover, if conditions are such that one vessel does not fill before the other is emptied, surging is again encountered.
It is highly desirable that a dredge be capable of operation in a wide variety of varying conditions, particularly with respect to the depth of the lake, stream, pond, or other body being dredged. Thus, the hydrostatic head at the material intake ports of the pressure vessels may vary over a wide range. With prior art control systems, particularly those referred to above, in which effective operation is dependent upon the relation between the filling and discharge rates of the pressure vessels, the depth range for the dredge is severely limited and many of the potential advantages of the pressure displacement pumping apparatus cannot be effectively realized. In particular, for effective dredging operations it is essential that a filled pressure vessel always be available to begin discharge whenever one of the other pressure vessels in the system approaches empty condition.
In very shallow streams and ponds, and in swampy areas, known pressure displacement pumps may be impractical for dredging because they are dependent upon hydrostatic pressure to fill the pressure vessels. Thus, if the body being dredged has a depth even minimally less than the height of the pressure vessels, the vessels fill very slowly and cannot be filled completely. Indeed, filling of the vessels is usually too slow for efficient operation unless a substantial head, well above the height of the pressure vessels, is available. On the other hand, if this difficulty is overcome, pressure displacement dredges can be more effective and efficient than mechanical dredges in swamps and other shallow bodies of water as well as in deeper bodies.